1. Field of the Invention
The present invention relates to improvements in spectrometers for electron probe X-ray analyzers, and more particularly to a novel multi-crystal positioning mechanism for such instruments.
2. Description of the Prior Art
The electron probe microanalyzer is an instrument for X-ray spectrochemical analysis of small areas on the surface of a solid specimen. It consists, basically, of three components: (a) an electron optics system to focus a beam of electrons on the specimen; (b) X-ray optics to analyze the X-rays emitted by the specimen under electron bombardment; and (c) a viewing system such as an optical microscope to aid in the selection of the exact area to be analyzed.
In an instrument of this kind such as that disclosed in the U.S. Pat. of Wittry, No. 3,107,297, the electron optics system and the viewing system are coaxial, and the emergence angle of the X-rays sensed by the X-ray analysis system is maximized in order to provide a desirable line-to-background ratio and to minimize the effects of surface irregularities. The resultant compacting of these components gives rise to special problems in the design of the X-ray optics of such an instrument.
Crystal diffraction, which is used for wavelength dispersion in such an instrument, depends upon the property of a crystal to diffract only particular X-ray wavelengths for each angular setting of the crystal with respect to the X-ray source. As that angle, called the Bragg angle, is changed, the wavelength diffracted will change, but a given crystal refracts efficiently only within a limited Bragg angle range. Therefore, in order to provide for the detection of X-rays within a wide band of the X-ray spectrum, a plurality of crystals of different characteristics, each adapted to efficiently diffract a different portion of the spectrum, is employed.
Since the crystals in such instruments necessarily are located in an evacuated section, various arrangements for effecting an interchange of crystals without breaking the vacuum have been proposed. These have included mounting two crystals on their holders back-to-back so that by a remotely controlled device either may be substituted for the other in analyzing position by rotation of the assembly through a 180.degree. angle. However, two crystals usually are not sufficient to permit the scanning of a sufficient portion of the X-ray spectrum. Therefore, it has been proposed to arrange a larger number of crystals on their holders on the periphery of a cylindrical mounting rotatable under remote control to bring any of the crystals into analyzing position. Such an arrangement, however, limits the angle through which any crystal so mounted may be rocked, since the viewing optics assembly embodied in such apparatus may, in many apparatus configurations, mechanically limit the motion of the crystal holder, preventing rocking of the crystal to as low a Bragg angle as may be desirable.
The primary object of the present invention is to overcome the deficiencies of prior arrangements of this kind and to provide a structure by means of which a series of crystals may be interchanged in analyzing position by remote control while permitting them to be rocked to as low a Bragg angle as desired.